1. Field of the Invention
The present invention relates to a projection image generation apparatus, program and method for generating a projection image by projecting a three-dimensional tubular anatomical structure in three-dimensional data onto a two-dimensional plane.
2. Description of the Related Art
In recent years, high-quality three-dimensional images (volume data) became used in image-based diagnosis as medical equipment (for example, a multi-detector CT, or the like) advanced. Volume data are composed of many two-dimensional images, and the information amount of the volume data is large. Therefore, in some cases, a doctor needs a time to find a desired observation region to diagnose a patient. Therefore, an improvement of a characteristic of recognizing a whole organ and a lesion was attempted by extracting and displaying an organ of interest by MIP, VR, CPR, or the like, thereby improving the efficiency of diagnosis.
Especially, Curved Planar Reconstruction (CPR) is known as an effective display method for observing a tubular structure in volume data (for example, please refer to A. Kanitsar et al., “CPR-Curved Planar Reformation”, VIS 2002. IEEE, pp. 37-44, 2002 (Non-Patent Document 1)). The CPR generates an image by sequentially reconstructing, along a path that a user wants to display, projection surfaces in directions orthogonal to the path. However, when tubular structures of branching plural paths are displayed at the same time, there is an unobservable region in some cases, because the paths overlap each other. Therefore, an Untangle CPR method has been proposed (for example, please refer to A. Kanitsar et al., “Advanced Curved Planar Reformation: Flattening of Vascular Structures”, VIS 2003. IEEE, pp. 43-50, 2003 (Non-Patent Document 2)). The Untangled CPR method displays an image by branching paths based on a certain rule in such a manner that the paths do not overlap each other.
When blood vessels are extracted from volume data and projected onto a CPR image, cross-sections of the blood vessels are displayed on the CPR image. Therefore, the CPR image is appropriate to observe a stenosis region of a blood vessel, which is caused by a plaque or the like. However, in the method of Non-Patent Document 1, an unobservable region is generated in some cases, because paths overlap each other. Meanwhile, in the method of Non-Patent Document 2, paths do not overlap each other. However, when a three-dimensional branching portion of blood vessels is expanded two-dimensionally, the blood vessels are displayed in such a manner to branch based on a predetermined rule. Therefore, a corresponding relationship with an actual anatomical position becomes unclear, and even if a stenosis region in a blood vessel is recognized in a CPR image, there is a problem that it is difficult to recognize at which position the stenosis has occurred in actual coronary arteries.